It is already well known to the one skilled in the art that relatively small molecules can be captioned by larger molecules which may be monomeric or polymeric, having an intramolecular or an intermolecular cavity. The size of such combinations of at least two different molecules, the host molecules containing guest molecules, is in the submicron range as mentioned above, namely between 50 and 500 nanometers.
Submicron particles are for example disclosed in the following patents: BE-A-808,034, BE-A-839,748, BE-A-869,107, FR-A-2,504,408, EP-A-0,275,796, EP-A-0,349,428, U.S. Pat. No. 5,718,905, WO93/25,194 and FR-A-2,681,868.
Belgian Patents No. 808,034 and 839,748 describe submicron particles formed by polymerization of monomers including acrylic or methacrylic acid derivatives. Micellar and/or interfacial polymerization of the various monomers generates particles of size under the micron scale. These systems may encapsulate active substances. The particles form stable aqueous colloidal suspensions allowing such the particles to carry biologically active molecules for medical administration. However, the high degree of stability and the lack of a suitable route for biological elimination of the polymeric acids give rise to a serious drawback in their use, in that they are retained within body tissue or within body cavities leading to possible side reactions.
This drawback is partially overcome according to patent BE-A-869,107 where biodegradable nanoparticles containing a biologically active molecule are cited. The polymers used are alkyl-cyanocrylate based copolymers of known biocompatibility. The main drawbacks of these systems arise from the toxicity of degradation products under physiological conditions as well as the methodology of the encapsulation of active substrates. Under the procedure described, a highly dense polymer lattice is the basis of the formation of the particles. Surface adsorption of molecules is used in the transport and hence incorporation levels are low. Secondary problems arise from the control of the polymerization reaction, which may leave either soluble monomers or soluble short oligomers, and these compounds may subsequently be leached from the matrix. The purification processes used are both highly time consuming and expensive.
Proteins, such as albumin, have been used to prepare nanosystems by thermal denaturing (Kramer, P. A. J., Pharm. Sci., 63, 1646, 1974) or by salt or solvent denaturing of proteins such as gelatin in solution (Martey et al., Aust. J Pharm. Sci., 6, 65, 1978 or Pharm. Acta Helv., 1, 53, 1978). The denaturated protein dispersion are subsequently reticulated. In the first case, the need to pre-disperse the protein in an oil-water emulsion requires use of secondary surfactant and also sonication. In the second case large amounts of inorganic material must be removed prior to use. In both systems excess, toxic, aldehyde must be removed.
EP-A-0,275,796 and EP-A-0,349,428 disclose nanoparticles that are prepared by solvent diffusion methods from two non-miscible systems. However, the protein-based nanosystems described in EP-A-0,349,428 require the use of highly specific conditions, limiting their application.
French patent FR-A-2,551,072 describes micrometric capsules as a form of sustained-released pharmaceutical system, which are prepared from polyol esters. However, such microparticles are not suited to intravascular administration without entailing medical risk.
Several patents, such as U.S. Pat. No. 5,718,905, WO 93/25194 and FR-A-2,681,868 describe the use of modified cyclodextrins as base material for the nanospheres. They have the advantage of being biodegradable, their administration is followed by release of the active molecule, and it is possible to obtain biodegradability, which is suitably controlled by making use of modified cyclodextrins which differ from each other in the nature of the substituent groups used. Such modified cyclodextrins and the preparation thereof are especially described by Pin Zhang, C. C. Ling, A. W. Coleman, Parrot-Loppez and H. Galons in Tetrahedron Letters 32, No. 24, 2679-70, 1991.
However, all the cyclodextrin based nanosystems share a number of problems; firstly high cost of synthesis and production as the initial modifications all require a vacuum drying of the cyclodextrins at 120° C.; secondly the use of toxic reactive material, including pyridine, iodine, sodium azide, secondary alnines which may remain trapped in the molecular cavities; thirdly biodegradation may liberate the parent cyclodextrin known to be highly hemolytic. No evidence has been presented that this hemolytic activity is absent in the modified species.